This invention relates to an image display device, and particularly to a modulation mask for a flat cathodoluminescent image display device.
One form of a flat image display device which has been developed includes a multiplicity of cells. Each of the cells includes all the necessary components for forming at least a single element of an image display. Typically, each cell includes a source of electrons hereinafter referred to as the cathode, means for modulating a flow of electrons from the electron source, means for accelerating and focusing the flow of electrons, and a cathodoluminescent screen excitable by the accelerated flow of electrons. The device is operated by suitably addressing the cells in a desired sequence, e.g., a typical television scan.
In order to form a display having desirable characteristics, the flow of electrons must be accurately modulated. Typically, on-off modulation of a cell can be easily accomplished. However, gray-scale modulation, i.e., a selective gradation of the number of electrons permitted to strike the screen, is much more difficult to achieve. This is especially true in those circumstances wherein cathodoluminescent flat panel display schemes should simultaneously satisfy the requirements of about 1 percent element-to-element uniformity, high color purity, simple drive circuit requirements, low cost, and ease of construction. In addition, in such a flat image display device, large area cathodes generally have nonuniform output currents and require a modulation scheme using sampling and control of charge rather than control of current, to display uniformity.
Thus, the extended nature of the cathode in such a flat image display device can necessitate at least one charge sensing electrode for each one of the elements per display line, e.g., about 1800 to 2200 per line for a color display. The extended cathode also requires a given modulating electrode to provide access to every one of the approximately 500 display lines, i.e., each modulating electrode should have a length equal to the full image height. In a simple vertical charge sensing grid system of modulation, the modulating electrode and the charge sensing electrode are one and the same. However, this approach imposes a fundamental lower limit on the charge sensing electrode capacitance since the modulating electrode must extend for the full panel height if it is to modulate all 500 lines. In addition, the electrode must be a sizable fraction of the picture element width, if charge sensing is to be accurate and/or if line source current demands are not made excessive. The fundamental lower limit on the electrode capacitance in such a scheme results in a useless and excessive power loss in charging the modulating electrodes since line sources generally require relatively high voltages for modulation. Accurate sensing in such a scheme would require greater than an order of magnitude more line source charge than is necessary to achieve desired brightness levels.
Therefore, it would be desirable to develop a means for modulation i.e., a charge sensing modulation mask, in a flat image display device which can form a display having desirable characteristics without demanding an excessive amount of line source charge.